Even if it means destroying the very world he fought so hard to save.

But I did it on my old computer, allowing me to keep my pacifist playthrough intact while still being able to experience everything the game had to offer without permanently corrupting my save file. Take that, game!

It would be entertaining if they could implement it somehow to maintain across save files via steam mechanics....

Say like award an achievement for a genocide run...call it CONGRATULATIONS and then Steam simply does a check for the achievement, no matter the PC it is played on to determine it.

I'm going to use an allegory to explain what I felt the game treats each playthrough as. One thing that needs to be accepted as a given is the idea of multiple universes. Now I want you to imagine that our universe is one possible result of a playthrough of a video game--or part of an ongoing playthrough. The player is a being that can travel from one universe to another and controls one person in any given universe. We are all NPCs. Now imagine that the player has encountered a scenario they didn't want to happen. The player relinquishes control of their character in our universe (possibly because their character died) and loads from an earlier save point. The player is no longer present in our universe. In that case, what becomes of our universe?

In the battle with Sans, Sans points out timelines just flat-out ending, which implies that when the player reloads, the universe they abandoned effectively ceases to exist. I felt that that was the argument being put forward by Sans (as well as what Flowey was implying would happen if we started a new playthrough after completing a "true pacifist" playthrough. Technically, that makes sense; any file that gets deleted is presumably gone forever. However, the argument begins to strike me as less reasonable once we take this idea farther. Does anything happen in our universe when the player isn't playing? For example, suppose the player just saved the game and has taken a break. The game implies that nothing happens when the player isn't playing, and again, that makes sense. The problem here is that such reasoning also implies time stops once the player has beaten the game. Sure, some games have playable epilogues, but even those can only keep the player's attention for so long. In other words, the universe will eventually just abruptly stop no matter what the player does. That's the conclusion I felt the game had reached: the universe only functions when the player is present; a universe with no player is perpetually frozen in time.

The main reason why I can't accept that conclusion is rooted in separation between gameplay and story. In a lot of games, it is implied that, story-wise, the universe continues functioning without the player's influence. In addition, what about beating the game? Does the universe just stop functioning once everyone's had their happy ending? Story-wise, that's quite the downer ending, don't you think? Even Undertale falls victim to this problem despite trying very hard to make gameplay and story one and the same. The True Pacifist ending is supposed to be the "and they lived happily ever after" ending. If the universe only functions when the player is there to influence it, though, then it's not so much "and they lived happily ever after" as it is "and they lived happily until the universe suddenly stopped existing." When that's kept in mind, a "true pacifist" playthrough becomes less about giving everyone a happy ending and more about giving everyone one final moment of happiness before everything suddenly ends.

Of course, that's not even getting into the matter of what happens before the player has any control over the universe. For there to have been any backstory, the universe must have been able to exist without the player's influence. However, once the player is present, any universe the player abandons is gone forever? There is really only one conclusion I can see that allows all that to make sense: The player is destined to destroy any universe they enter.

Once you actually think about why games have stories in the first place, that conclusion seems to be completely contrary to the tone of most games I've played. The player character is often supposed to save the world, not destroy it. Games are designed with a specific purpose. If all game universes are supposed to stop functioning once the player is no longer present, then a lot of stories from games suffer a serious shift in tone--often to a tone the developers wouldn't have wanted. In fact, I'm inclined to wonder if Mr. Fox wanted such a conclusion to be reached, considering the tone of the "true pacifist" ending. Undertale's story pointed me toward a conclusion that appears to contradict the purpose of a "true pacifist" playthrough.

In the end, my previous perception of game universes stands: story holds priority over gameplay. If the universe is implied to continue functioning after the player has beaten the game and stopped playing, then it continues functioning.

I had a different take on it.

After you loaded the game up after beating the Pacifist ending you get a special message from Flowey. It talks about how you still have the power to ruin everything by resetting that happy ending.

"Let Frisk live their life"
"Let Frisk be happy"

And it talks about how Peace and Prosperity are currently reigning throughout the land. It's all things to do with the future specifically, given the game's one-save slot, by NOT playing the game. It to me implies that the universe would go on.

As for Sans' line about timelines disappearing...I took this to mean other completed Genocide runs rather than just when the game stopped playing.